1. Field of the Invention
The present invention is related to an autofocus actuator, and in particular to an autofocus actuator which can be used in a compact electronic device equipped with a camera such as a digital camera or portable telephone or the like.
2. Description of the Prior Art
In digital cameras and the like, an actuator which can move lenses in the optical axis direction by the interaction between a magnetic field due to a permanent magnet and a magnetic field generated by electric current flowing through a coil is used for the purpose of carrying out autofocus and zoom. Further, with the recent development of megapixel image taking elements having a pixel number of one million pixels or more, there has been a demand for an autofocus function even in cameras installed in portable telephones.
A well-known prior art autofocus actuator is disclosed in Japanese Laid-Open Patent Application No. 2004-280031. As shown in FIG. 20, a prior art autofocus actuator 100 is equipped with a yoke 130 having a roughly U-shaped cross-section, a permanent magnet 140 attached to the outer wall inner surface 132 of the yoke 130, a holder 110 (carrier) provided with a lens assembly 105 at a center position, a coil 120 mounted in the holder 110, a base 185 in which the yoke 130 is mounted, a frame 180 which supports the base 185, and two leaf springs 160U, 160L which support the holder 110. The holder 110 is inserted into a through hole formed by an inner cylindrical portion 131 of the yoke 130. The two leaf springs 160U, 160L are the same and are arranged to sandwich the holder 110 from above and below. Further, the two leaf springs 160U, 160L function as a feeder circuit of the coil 120. By balancing the current value applied to the coil 120 and the restoring force of the two leaf springs 160U, 160L, it is possible to control the amount of movement of the lens assembly 105 mounted to the holder 110.
As an example of the external dimensions of an autofocus actuator mounted in a portable telephone, the length is approximately 10 mm, the width is approximately 10 mm, and the thickness is approximately 5 mm.
In the prior art autofocus actuator 100 described above, if the holder 110 and the inner cylindrical portion 131 of the yoke 130 make contact with each other, there is the problem that it becomes impossible to move the lenses with good precision.
Further, if the space between the holder 110 and the inner cylindrical portion 131 of the yoke 130 is made large so that the holder 110 and the inner cylindrical portion 131 of the yoke 130 do not make contact with each other in order to solve this problem, there is also the problem that the shock resistance of the autofocus actuator 100 is degraded.
For example, in the case where a portable telephone falls from a height where it was being used, the space between the holder 110 and the inner cylindrical portion 131 of the yoke 130 needs to be made about 0.1 mm in order to ensure sufficient shock resistance.